1. Field of the Invention
The present invention relates, in general, to scanning probe microscopes and, in particular, to probes for scanning probe microscopes.
2. Background Description
A scanning probe microscope, such as a scanning tunnelling microscope, positions a conducting tip or probe extremely close, such as ten angstroms, to the surface of a conducting sample. A voltage applied between the probe and the sample causes a current referred to as a tunnelling current to flow between the sample and tip.
Such microscopes include a positioning device to control the X-Y-Z scan of the probe or tip over the surface of the sample. As disclosed in U.S. Pat. Nos. 4,785,177; 4,871,938 and 5,325,010, a plurality of tubular, piezoelectric electrodes are mounted on the scanning head and engage the surface of the sample, typically by a ball mounted on the end of each electrode. The scanning tip or probe is mounted in another piezoelectric electrode and held at a constant separation distance from the surface of the sample. The application of electric voltage to the electrodes causes deformation of the piezoelectric electrodes thereby providing a precise X-Y-Z control of the position of the scanning head and tip relative to the surface of the sample.
Despite the close separation distance of approximately ten angstroms between the probe and the surface of the sample, it is imperative that the probe remain separated from the sample. Any contact of the probe and the sample generally will damage the probe sufficiently to render it inoperative. As a result, it becomes frequently necessary to replace the probe in a scanning probe microscope. At best, this is a difficult procedure particularly when the microscope is mounted in a vacuum chamber. A remote manipulator extending into the chamber may be required to remove the probe from the scan head and to mount a new probe therein. The close arrangement of components within the vacuum chamber and the extremely small size of the probe make axial alignment of the probe with the probe holder in the scan head extremely difficult.
Further, it is important to provide the proper length when mounting a new probe in the scan head as probe length must be within the coarse approach range of the scan head. While certain of these difficulties can be eliminated by opening the microscope chamber, such is not desirable since it usually requires at least two days to reattain the desired vacuum level.
Another problem encountered in the use of scanning probe microscopes involves the repeatable mounting of the sample relative to the probe. Frequently, the sample is mounted in a holder which is inserted into an open-ended, U-shaped slot in a guide bracket mounted on a support base. The slot in the guide bracket limits the insertion distance of the sample holder into the guide bracket; but still enables the sample holder to be freely rotated. This makes it extremely difficult, if not impossible to obtain the same probe position on consecutive measurements on the same sample after removing and reinserting the sample in the microscope. Such sample guides are not precise enough to enable removal and insertion of a sample into the same scan spot.
Thus, it would be desirable to provide a probe holder for a scanning probe microscope which provides accurate alignment of a replacement probe with a probe holder in the microscope scan head. It would also be desirable to provide such a probe holder which can utilize the same manipulator used to mount the sample in the microscope chamber. It would also be desirable to provide a probe holder for a scanning probe microscope which provides quick and easy replacement of a probe in the scan head. It would also be desirable to provide a probe holder which is self-aligning during probe replacement. It would be desirable to provide a scanning probe microscope with an alignment mechanism which provides repeatable, accurate positioning of the probe holder with respect to the probe mounting socket in the scan head. It would also be desirable to provide an alignment device for repeatable, accurate positioning of a sample holder with a scan head.